1. Field of the Invention
The present invention relates to an image forming apparatus that conveys a recording member during formation of a toner image or after the toner image is formed by a toner-image forming unit, by nipping the recording medium in a transfer nip formed by a pair of conveying members abutting against each other while making a surface movement.
2. Description of the Related Art
In image forming apparatuses related to the present invention, a sheet-like recording member inserted into a transfer nip formed by a pair of conveying members such as a combination of a photoconductor and a transfer roller, or a pair of fixing rollers can wind around one of the conveying members due to viscosity of a toner image on the surface thereof or electric charge. If severe winding occurs, recording paper can be wound into a unit that includes the conveying members, and it requires maintenance work by a service engineer. Specifically, the image forming apparatus generally has a door for exposing a conveying path to the outside according to need, and the recording paper as the recording member blocked in the conveying path can be easily removed from a space opened by the door. However, the recording paper wound in a process unit containing the photoconductor as the conveying member or a fixing unit containing the fixing roller as the conveying member cannot be removed unless the unit is disassembled. Because a certain degree of knowledge is required for unit disassembling, its user cannot handle the problem and needs help of a service engineer.
Winding of the recording paper into the conveying member is largely associated with wearing of the conveying member. The surface of the conveying member in an initial state has excellent smoothness, and can demonstrate excellent releasability with respect to the toner having viscosity or electric charge. Therefore, winding of the recording paper with respect to the conveying member hardly occurs. However, when the smoothness of the conveying member is lost with long time use (when the conveying member wears), winding can easily occur.
In the fixing unit that fixes the toner image on the recording paper, the toner image is softened by heating to increase the viscosity thereof. Therefore, winding of the recording paper around the fixing roller tends to occur. Accordingly, there is a type of an image forming apparatus in which the recording paper advanced close to wind around the fixing roller is forcibly peeled off from the fixing roller by a separation claw installed adjacent to the fixing roller. However, even with the separation claw, winding of the recording paper into the unit cannot be completely avoided. When adhesion between the toner image and the fixing roller considerably increases due to formation of a full-page solid toner image, the recording paper can slip through between the separation claw and the fixing roller.
Therefore, in Japanese Patent Application Laid-open No. 2007-108618, the inventors have proposed an image forming apparatus as described below. That is, the image forming apparatus detects behavior of recording paper fed out of an outlet of a fixing nip formed by a pair of fixing rollers, based on a time-series detection result obtained by a distance sensor that detects a distance between the recording paper and the sensor itself. The image forming apparatus then determines a wearing of the fixing roller based on the behavior, an image area ratio of the toner image, and a thickness of the recording paper, and urges the user to replace the fixing roller based on the result. When the fixing roller wears and the recording paper easily tends to wind around the fixing roller, the recording paper discharged from the outlet of the fixing nip is not separated promptly from the fixing roller, and starts to exhibit a behavior following the surface of the fixing roller. When the recording paper begins to exhibit such a behavior, the user is urged to replace the fixing roller, thereby enabling to prevent before an occurrence of winding of the recording paper due to continuous use of the worn fixing roller.
The following is the reason why the image area ratio of the toner image and the thickness of the recording paper are used in addition to the behavior of the recording paper fed out of the outlet of the fixing nip as the parameter for determination of the wearing of the fixing roller. That is, easiness of winding around the fixing roller of the recording paper fed out of the outlet of the fixing nip is associated with an image area of the toner image formed on the recording paper and stiffness of the recording paper other than surface smoothness of the fixing roller. Even if the wearing of the fixing roller (degradation degree of the surface smoothness) is the same, the behavior of the recording paper after passing the outlet of the fixing nip becomes different between a case having a relatively large area and a case having a relatively small area of the toner image, which demonstrates the viscosity and an electrostatic force. As the area of the toner image becomes larger, the time required for separation of the recording member fed out of the fixing nip from the surface of the fixing roller (hereinafter, “recording paper separation time”) becomes longer. Further, the behavior of the recording paper fed out of the outlet of the fixing nip becomes different between a case of having a relatively strong stiffness and a case of having a relatively weaker stiffness of the recording paper. As the stiffness of the recording paper becomes weaker, the recording paper separation time becomes longer. Therefore, the wearing of the fixing roller is determined, comprehensively taking the image area of the toner image and the thickness of the recording paper into consideration in addition to the behavior of the recording paper.
However, in the image forming apparatus of Japanese Patent Application Laid-open No. 2007-108618, there is still a room for improvement as follows. That is, the thickness and stiffness of the recording paper have a certain degree of correlation; however, the correlation is not a strict proportional relationship. Even with the recording paper having the same thickness, the recording paper having a wider width demonstrates a stronger stiffness. Nevertheless, the wearing of the fixing roller is determined, regarding the thickness of the recording paper as the stiffness. Accordingly, a slight discrepancy has been generated between the determination result and the actual wearing.